The invention relates generally to satellite control systems and, more particularly, to a method and apparatus for managing the configuration of a fleet of satellites in low earth orbit.
Satellite communications systems which comprise one or more communications satellites that travel about the globe in non-geosynchronous orbits and one or more ground stations located in various regions of the world are known. A given satellite and a given ground station can connect to each other to effect communications when they are in view of each other. However, they generally will not be able to communicate properly unless they are configured to operate according to the same communications protocol.
Different communications protocols are used in different regions of the world. A ground station located in a particular region is configured to operate according to that region's communications protocol. Since the satellites are contemplated to fly over, and communicate with, ground stations in different regions, it is important that the satellites can be reconfigured, on-the-fly, according to the local communications protocols of the regions they fly over and the ground stations located therein.
Direct command control systems whereby a satellite can be reconfigured by executing commands transmitted to the satellite from a ground station are known. However, in some systems, the satellites are receptive to such direct commands from some, but not all, ground stations. For example, a satellite may be receptive to direct command by ground stations located in the United States, but not those located in other parts of the world.
In situations where a satellite needs to adopt only two protocols throughout its orbit, one for use when flying over the command-receptive region and another for all other regions it flies over, the foregoing limitation would not present a significant obstacle to operation. The satellite could simply be reconfigured for the required protocol as it approached and departed from the command-receptive region.
In practice, however, it is likely that a satellite's orbit will take it over several different regions using several different protocols while the satellite is out of sight of, and therefore cannot receive commands from, a ground station in a command-receptive region. To accommodate such situations, the satellite must be reconfigurable by means other than direct control in order to maximize its availability as it travels throughout its orbit.
One known method for reconfiguring a satellite by means other than direct control involves using known information about the satellite's orbit to calculate which regions it will be flying over at which times, and to thus determine which communications protocol it must be configured for at a particular time. This information can be used to generate a command set which can be uploaded to the satellite from a ground station in a command-receptive region. The satellite's on-board computing facilities can then execute the appropriate command set at the appropriate time to reconfigure the satellite, as needed, as it travels throughout its orbit.
While the foregoing method is effective, it requires that the times that the satellite will enter and exit the regions of interest be precalculated on the ground and uploaded to the satellite. Based on the limited computing and storage resources on board the satellite and on other factors, such information generally needs to be calculated and uploaded to the satellite regularly. It may be necessary to upload hundreds of time-based command sets to each satellite every day. This places a significant burden on the ground control staff and hardware, as well as on the limited computing and storage resources on board the satellite. Furthermore, the command upload process is subject to error. The greater the number of commands that must be uploaded, the greater the potential for errors to occur.
Accordingly, it would be desirable to provide a method and apparatus by which a satellite can reconfigure itself autonomously to adopt the local communications protocol of the regions it flies over, which places a minimal burden on the satellite hardware and control staff.